cmd/gc: Don't calculate second value in range if it is blank.
cmd/gc: Don't calculate second value in range if it is blank.

Low hanging fruit optimization. Will remove an expensive copy if the range variable is an array.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/6564052

gnanderson/go,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,gnanderson/go,dsriram/golang,tv42/old-go,apparentlymart/golang,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,tv42/old-go,dsriram/golang,gnanderson/go,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,gnanderson/go